The generation of small devices often involves an alignment of multiple material layers, e.g., two or more material layers, with the assistance of an imaging system. Alignment requirements generally become more exacting as feature sizes on the devices shrink, requiring imaging systems with increased lateral resolving power. Some degree of spacing between the material layers is generally required during the alignment process. In some contexts this spacing is temporary, as in the case of mask and substrate alignment in semiconductor device fabrication using proximity lithography. In other cases this spacing may be more permanent, as in the generation of micro-electromechanical machine (MEMS) devices, nanostructures, or the like.
Because there is often an inverse relationship between the resolving power and the depth of field of an imaging system, one issue brought about by shrinking feature sizes is that the spacing between material layers may begin to exceed the depth of field of the imaging system used in the alignment process. This can bring about difficulties in proper alignment sensing. This issue can be compounded in the context of more advanced device fabrication techniques or more complex MEMS or nanostructural devices that may require concurrent aligning of more than two material layers. Another issue arising out of these concerns or other aspects of shrinking feature sizes is that it may be frustrating, tiresome, or otherwise undesirable for a human to be involved in the alignment sensing or control process.